We are attempting to define in biochemical terms, the functional activities of various leukocytes. These are neutrophilic granulocytes and macrophages from several species, e.g. mouse, rat, guinea pig, and man. The work is divided into two parts: study of the modes of ingestion of particles, and study of the mechanism(s) of intracellular microbicidal action. We are examining the membranes of selected leukocytic phagocytes particularly with respect to ectoenzymes and other potential markers that would allow us to define areas involved in phagosome formation, and we are exploring the various modes by which different leukocytes kill ingested microorganisms. Specifically, in the latter context, the roles of oxidative processes involving peroxide and superoxide are under investigation. We are paying special attention to the "activation" of macrophages, especially in terms of the definition of "activation", and the ways by which activated macrophages manifest increased bactericidal activity. The overall objective, as before, is to provide information on the manner in which leukocytes combat infection.